File:The cell and protoplasm (1940) (20400730759).jpg

Title: The cell and protoplasm ..
Identifier: cellprotoplasm00amer (find matches)
Year: 1940 (1940s)
Authors: American Association for the Advancement of Science; Moulton, Forest Ray, 1872-1952
Subjects: Cells; Protoplasm; Cytology; Cytoplasm
Publisher: Washington, Pub. for the American asscoiation for the advancement of science by the Science press
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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STRUCTUHAL UNITS IN t'ELLlTLAK PHYSIOLOGY 201 has a minimum at a definite distance of equilibrium. Tlie )iliysieal si<iiiificaiice of this equilibrium depends, however, on the energy inyolved. For two spherical i)ar- ticles of the dimensions of a large protein molecule, diameter 100 A, this minimum is of the order of 0.1 of the thermal energy, and consequently no e(inilil)rium results, the total etfect of the forces being to pre- vent agglutination. The situation is very different, however, with larger particles and especially with elongated, needle- or plate-like particles. In this case the en- ergy of equilibrium may be much greater than h-T, with the result that a stable ecjui- librium occurs at ordinary temperatures. 4-
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Fk;. 2r. Two similar i);trtick'.s close together; repulsive forces. This was first observed in tobacco-mosaic virus, where the long particles maintain themselves in a regular hexagonal array (Fig. 2), even down to dilutions of about 2 per cent dry matter; that is, when the particles are 800 A apart. In the case of flat particles the equilibrium is even more stable. The existence of these forces fur- nishes an explanation for the iridescent ferric-hydroxide gels, which consist of flat particles in equidistant layers that may be as much as 5,000 A apart, or of the order of the wave length of visible light, so that their regularities give an effect similar to that of butterfly wings. These long-range Langmuir-Levine forces are extremely sensitive to changes in the concentration or pH of the medium, and here we have a mechanism ideally suited for the develop- ments of easily transformable fluid and yet definite structures such as w^e observe in living cells. A characteristic feature of these forces is the appearance of spindle-shaped bodies, usually called factoids (Fig. 2). The shape of a factoid depends on the existence of orientation of the long particles on a free surface, and thus on the ))roduction of a surface tension which is different in the two directions of that surface. The shape can be ((nantitatively accounted for on this h3"pothesis. Roughly it nui.y be said that the spindle shape depends on the relative ease with which the surface can be bent on u 1 1 \ V ^ D V Fiii. ID. Potential fimctiou, Urdiiiates, potential IT; abscissas, distances r. an axis parallel to the long direction of the particles compared with that of bending it along the axis perpendicular to them. Such factoids have been often observed in living structures, the most striking case being that of the spindle found at certain stages of mitosis. Pfeiffer was the first to suggest that this spindle is probably a fac- toid, and that the fibers of which it is nor- mally supposed to be composed are arte- facts formed by the dehydration action of the fixing medium. The study of tobacco- mosaic factoids confirms this suggestion; in this case, also, fibers are formed which

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